What to ask when buying used speakers?

Lots of good advice here so far! If buying from a reputable dealer I wouldn't worry honestly, assuming they have a good return policy. If buying from a dealer, I would still like to know if the speakers were modified in any way. This can make them sound different than what you are expecting, but it can also lead to problems in the longer run. I've seen speakers with changes to the crossover or drivers which would put a lot of stress on the other drivers, but also once bought a pair where the previous owner drained all the ferrofluid from the tweeter to 'make them sound more detailed'.

I buy all my speakers used and I do always expect to have to do some small maintenance, such as glueing a surround or replacing ferrofluid. If you don't mind doing something like that with some pairs, there are real gems to be found for very little money (:

@mtbiker29 cats destroy grills, but I've also seen surrounds and cones with claw marks.

Speakers (and other components) I  have almost always bought dealer demo or used from a reputable dealer.  They have always stood behind all details of the product and sale.  No worry a that product had been misused.  Twice when I bought something i had not been able to demo and did not like the product, the dealers took them back within 30 days no hassle with full refund or very minimal fee.  My years of building relationships with dealers who I trust and now trust me, has proven very valuable.  I now get great deals and service.  I like supporting dealers.

I am glad the responses were helpful.

I, and I am sure other contributors will be interested in, is whether you do buy some used speakers, which ones, and how successfully.

Could you update in due course please?

Will do.

I'm happy for the folks that can spend 2.5x the used price to avoid the risk of used gear.  I, on the other hand, have only been able to have a taste of high end audio by buying used.  

Yeah, I've gotten lemon gear. But overall through the life of the hobby there's no question I have saved over 50% of the new budget by buying mostly used gear.

I agree with the advice above that the best protection is getting to know the seller.  For any significant purchase, I invest my time in ongoing dialog with the seller. Not only does that "make friends" and make the transaction more pleasant, but you can get a very good sense of the person, their knowledge and treatment of gear, and ultimately build trust with them.  

I also buy a lot of used gear from Andy at Saturday Audio in Chicago.  He stands behind everything and the gear is thoroughly checked out.  He's swapped out speakers for me when something was amiss.  This is the type of dealer relationship you will also want if you are building an ever-evolving system. 

One thing I would say is that buying some components from a smoker household may not matter, speakers are big and right in the room and pick up (and release in your house) the smell of smoke worse than most other components.  I remember having to wash some ML panels in my shower to make my room livable. 

Used gear, and navigating and negotiating...that's part of the fun for me! I have met a lot of really nice people, all walks of life, all layers of society. You'd be surprised what gear some guys who make $40K per year actually have!